This invention relates to a process for obtaining adipic acid from the by-product stream ("mother liquor") arising from the oxidation of technical mixtures of cyclohexanol/cyclohexanone using nitric acid and isolation of the principal quantity of adipic acid by crystallization, in which the acids remaining in the stated mother liquor are obtained as a mixture ("technical glutaric acid").
During the production of adipic acid by oxidation of cyclohexanol and/or cyclohexanone, in particular of technical mixtures of cyclohexanol and/or cyclohexanone using nitric acid, glutaric acid and succinic acid are formed as by-products. These by-products accumulate in the adipic acid crystallization mother liquor. A proportion of this mother liquor is thus discharged from the process in order to prevent by-product accumulation. Once nitric acid and water have been separated from this mother liquor, the remaining dicarboxylic acid mixture may be worked up by distillation to yield "technical glutaric acid". The resultant distillation residue contains predominantly adipic acid and carbonization products, together with metallic catalysts such as copper or vanadium.
The adipic acid contained in the dicarboxylic acid mixture (approximately 25 to 45 wt. %) is effectively lost as adipic acid, reduces the quality of the technical glutaric acid obtained, consequently impairing its suitability for use in the tanning material sector, and increases the amount of the distillation residue when the glutaric acid is worked up by distillation.
A series of processes has thus been developed to recover adipic acid. According to U.S. Pat. No. 4,146,730, addition products, which may be separated from the adipic acid, are formed from glutaric and succinic acid on the one hand and urea on the other. U.S. Pat. No. 3,818,081 describes the conversion of glutaric and succinic acid into the imides and the separation thereof from the adipic acid. According to European Patent Application 33,851, glutaric and succinic acid are converted by reaction with alkylamines into the corresponding amides, which may in turn be separated from the adipic acid. U.S. Pat. No. 4,442,303 discloses esterification of the dicarboxylic acids and separation of the acids in the form of their esters. U.S. Pat. No. 4,014,903 discloses simple cooling crystallization to recover adipic acid from the by-product discharge stream. However, the efficiency of such a process remains unsatisfactory for an industrial process because the quantity of adipic acid that may be obtained in this manner is small and, in particular, because (as is also confirmed in U.S. Pat. No. 4,254,283) much succinic acid is crystallized out together with the adipic acid, such that a further stage is necessary for the purpose of separating these two acids. As disclosed in U.S. Pat. No. 4,254,283, this further stage may, for example, proceed by converting the succinic acid into succinic anhydride which is then separated from the adipic acid by distillation.
The object of the present invention was thus to provide a novel process for the recovery of the adipic acid present in the acidic mother liquor, which, in comparison with the stated prior art processes, may be performed in a simple manner and yet yields adipic acid or "technical glutaric acid" of a comparatively elevated purity. This object could be achieved by means of the process according to the invention which is described in greater detail below.